(1) Field of the Invention
The present invention relates to thermoelectric conversion materials to be used for exhibiting so-called thermoelectric effects (direct energy conversion without relying upon moving parts) such as thermoelectric generation due to the Seebeck effect or thermoelectric cooling due to the Peltier effect. The invention also relates to a process for producing such thermoelectric conversion materials.
(2) Related Art of the Invention
The thermoelectric conversion such as thermoelectric generation and thermoelectric cooling by using a thermoelectric conversion material enables the production of a simplified energy direct conversion device which has no moving parts causing vibration, noise, abrasion, etc., is simple in structure with high reliability, possesses long service life, and is simple to maintain. For example, the thermoelectric conversion is suitable for directly obtaining DC electric power by combustion of a variety of fossil fuels, etc. and for controlling the temperature without use of a cooling medium.
In evaluating the performances of the thermoelectric conversion materials, an electric power factor Q and a performance index Z (also commonly known as "Figure-of-merit") expressed by the following equations are used. EQU Q=.sigma..alpha..sup.2, Z=.sigma..alpha..sup.2 /K
in which .alpha. is a Seebeck coefficient, .sigma. is an electrical conductivity, and .kappa. is a heat conductivity. As to the thermoelectric conversion material, it is desired that the performance index Z is large, that is, the Seebeck coefficient is high, the electric conductivity .sigma. is high, and thermal conductivity .kappa. is low.
When the thermoelectric conversion material is used for thermoelectric cooling or thermoelectric generation, particularly when the thermoelectric conversion material is used as a thermoelectric cooler for a high-temperature generating member or a thermoelectric generator for the utilization of waste heat, it is desired that the thermoelectric conversion material has a high Figure-of-merit Z of not less than 3.times.10.sup.-3 [1/K] as the thermoelectric performance, and operates stably in a use condition for a long time period. Further, it is also desired that the thermoelectric conversion material has sufficient heat resistance and chemical stability in a temperature range of not less than 300.degree. C. Furthermore, if a cooler utilizing thermoelectric cooling or a thermoelectric generator is mass produced for vehicles, etc., it is desired that a material to be effectively produced at a low cost and a process for producing the same are available.
Heretofore, tellurium based compounds such as Bi.sub.2 Te.sub.3,
Bi.sub.2 Sb.sub.8 Te.sub.15, and BiTe.sub.2 Se have been known as thermoelectric conversion materials having high performance indexes Z=3.times.10.sup.-3 [1/K]. Further, thermoelectric conversion materials using Sb compounds such as TSb.sub.3 (T: Co, Ir, Ru), for example, thermoelectric conversion materials in which an impurity for determining a type of electrical conduction is added into a material having a main component of CoSb.sub.3 in its chemical composition are described in the following documents.
1) L. D. Dadkin and N. Kh. AbrikoSov, Soviet Physics Solid State Physics (1959) pp. 126-133 PA0 2) B. N. Zobrinaand, L. D. Dudkin, Soviet Physics Solid State Physics (1960) pp. 1668-1674 PA0 3) K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto and T. Koyanagi, American Institute of Physics (1995) pp 226-229.
However, although the thermoelectric conversion materials made of the Te based compounds represented by Bi--Te series have large performance indexes Z of about 3.times.10.sup.-3 [1/K] at near room temperature, their characteristics are deteriorated at not less than 300.degree. C., so that the use temperature is unfavorably limited to a large extent. Further, since a volatile component such as Te or Se is contained in the compositions of the materials, these thermoelectric conversion materials unfavorably have low melting points and lack chemical stability. Furthermore, since a producing process is complex, the characteristics are likely to vary due to changes in the composition, and the intended materials cannot be unfavorably effectively mass produced. In addition, a poisonous element (Te) is contained in the starting materials, and since an expensive starting material having a high purity is needed, an inexpensive product cannot be offered.
As to thermoelectric conversion materials composed mainly of Sb compounds such as TSb.sub.3 (T: Co, Ir, Ru), for example, CoSb.sub.3 in their chemical compositions, it is known that raw materials are relatively inexpensive and contain no poisonous elements, and chemically stable even in a temperature range of not less than 300.degree. C. Although the use temperature of the thermoelectric conversion material having the chemical composition of CoSb.sub.3 is wider than that of the Bi--Te based material, the former is inferior to the latter in that the electric conductivity is lower and the Figure-of-merit (Z&lt;1.times.10.sup.-3 [1/K]) is far smaller.
It is considered that the formerly known thermoelectric conversion material having the chemical composition of CoSb.sub.3 has only a cubic CoSb.sub.3 crystalline phase as its constituting chemical phase, and that other crystalline phases (CoSb, CoSb.sub.2, Sb) function to deteriorate the thermoelectric characteristics.
However, it is known that when such a thermoelectric conversion material is obtained by melting CoSb.sub.3, phases other than CoSb.sub.3 (e.g., CoSb, CoSb.sub.2, Sb) (e.g., CoSb, CoSb.sub.2, Sb) are precipitated during solidification. In order to attain a single CoSb phase, heat treatment needs to be effected at around 600.degree. C. for about 200 hours. This unfavorably prolongs the producing period.
Further, according to a process for producing a thermoelectric material by grinding CoSb obtained through melting and sintering the milled powder, since a foreign phase precipitated during the melting, that is, CoSb and CoSb.sub.2 having densities higher than that of CoSb.sub.3 are converted in phase to CoSb.sub.3 during firing, the volume of the material increases to hinder the sintering. For example, a sufficiently densified thermoelectric conversion material has not been obtained even by hot press under the condition that the pressure was 5.times.10.sup.3 kg/cm.sup.2 and the temperature was 600.degree. C. (K. Matsubara, T. Iyanaga, T. Tsbouchi, K. Kishimoto and T. Koyanagi, American Institute of Physics (1995) pp. 226-229). As a result, a material fired under atmospheric pressure is brittle, and has conspicuously low electrical conductivity and extremely poor thermoelectric property.
As mentioned above, although the thermoelectric conversion material having the chemical composition of CoSb.sub.3 has the wider usable temperature range as compared with the Bi--Te based one, the former has the problems in terms of both the material characteristics and the production process. Therefore, there has been demanded a thermoelectric conversion material which is chemically stable and unlikely to be degraded, has excellent thermoelectric property and high strength in a wide temperature range of from room temperature to not less than 300.degree. C. Further, a simplified process for producing such a thermoelectric conversion material has been demanded.